Traditionally Time Division Multiplex (TDM) based traffic has been transported in synchronous or plesiochronous fixed sized frames, in technologies such as Synchronous Digital Hierarchy (SDH) and Plesiocronous Digital Hierarchy (PDH). The rate and size of frames remains the same regardless of the usage.
With the introduction of packet based traffic such as provided by Ethernet and IP network services, packets are sent from a transmitting node whenever there is data. This allows for statistical multiplexing of various streams.
A TDM frame with its fixed continuous repetition rate can be mapped onto a stream of packets. Each TDM frame holds N slots of data, of which a predetermined number of slots can be mapped into packet(s), which when done continuously forms a packet stream carrying the full TDM frame sequence over a sequence of packets. Such a mapping of TDM frames into packets will produce a fixed rate of packets regardless of the usage of the TDM slots.
One approach to build packets (illustrated in FIG. 1) from a repetitive TDM frame 101 is to view the TDM slots as a continuous slot stream 102, where every N'th slot is being marked as slot 0 in a frame. Within the slot stream 102 the slot numbers can be given by dead reckoning, just as with normal TDM transmission.
This slot stream 102 is then put into packets 103 by including P number of slots from the slot stream 102 into each packet. If the packet includes a slot 0, the first slot being a slot 0 is marked, as illustrated by the arrows 104 and 105 in FIG. 1. If the frame 101 is so small that a number of frames 101 can be held within the packet, then pointing out the first slot to be slot 0 is done by utilizing that the number of slots P allowed for implications of the other frames is known.
Use of sequence numbers on the packets, will allow for packets to be detected as missing as well as to sort them to overcome re-ordering such that the TDM frames 101 can be recovered properly at a receiving node.
Optionally Forward Error Correction (FEC) can be provided such as 1 or 2 dimensional FEC matrices, as being used in SMPTE 2022 data-streams.
When using a FEC correction a number of packets may be generated and transmitted, for instance: 9 data packets followed by a parity packet. When received in the receiving node, the packets are buffered, and if one packet is missing, as result of being dropped by the network, that packet may be re-created from the other 9 packets. This requires all the 10 packets to be buffered for restoration prior to releasing the data for further processing. This causes a delay, for a high packet rate, say 1000 packets a second, which amounts to 10 ms, where as a slower rate of 200 packets a second represents 50 ms of delay. With a varying packet rate, the delay will also vary for the same FEC configuration. More complex FEC configurations to achieve better redundancy will require more packets and hence longer delay.
The end result will be (excluding now the additional packages due to FEC) a packet stream 103 having a packet rate of fpacket=fframe*N/P. This packet rate will be independent on the number of actually allocated slots out of the N slots being setup in each TDM frame.
The problem with this is that the packet rate remains the same high number regardless of amount of capacity actually being in use in the TDM frame. For a TDM frame setup for 100 Mb/s a usage of only 10 Mb/s would still produce the packet rate to support the 100 Mb/s. This behavior is inflexible and rented capacity or use of infrastructure is less cost-efficient than desired.